Add doc8 rule and check doc/source files

doc8 is a linter for documents and used in openstack-manuals.
It is better to enforce document linters for simple checking.
This change is to add doc8 in tox file and fix line too long
in some files.

The current rules are as bellow:
- invalid rst format - D000
- lines should not be longer than 79 characters - D001
  - RST exception: line with no whitespace except in the beginning
  - RST exception: lines with http or https urls
  - RST exception: literal blocks
  - RST exception: rst target directives
- no trailing whitespace - D002
- no tabulation for indentation - D003
- no carriage returns (use unix newlines) - D004
- no newline at end of file - D005

Change-Id: I5b409fbfd95e05921310c6ecf4afea0220fb0bf0

test-requirements.txt

@@ -5,6 +5,7 @@ hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 
 beautifulsoup4>=4.6.0 # MIT
 coverage!=4.4,>=4.0 # Apache-2.0
+doc8 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 mock>=2.0.0 # BSD
 python-subunit>=0.0.18 # Apache-2.0/BSD